Throughout the project, the single-strand and double-strand forms of bacteriophage phi X174 DNA will be used as a model system to study the molecular action of ultraviolet (UV) and ionizing radiation on DNA. There are two major objectives of this proposal; first, to study the influence of the molecular environment of DNA (both chemical and enzymatic) on its radiosensitivity, and the types of damage produced by the radiation; and, second, to determine which types of radiation damage in DNA are biologically important with regard to mutagenesis and lethality. Ionizing radiation produces strand breaks and base damage in DNA. The chemistry of the strand breaks and and the types of base damage varies in response to changes in the chemical environment of the DNA at the time of irradiation. Experiments will be designed to study the response of damage produced under different radiation conditions to various repair systems, especially the induced repair system. Several agents are known which can introduce specific types of base damage, including alkylating agents (e.g., methyl methansulfonate) and osmium tetroxide (which only reacts with single-strand DNA). Exposure of DNA to near UV radiation gives a different spectrum of base damage than exposure to far UV. Mild heat treatment of alkylated DNA, or more extensive heat treatment of normal DNA, leads to depurination. Biological killing and mutagenesis in DNA damages by these types of agents will be studied, for the purpose of determining the biological consequences resulting from base damages of various types. The broad objective of this project is to determine the mechanism(s) of killing and mutagenesis by radiation with particular emphasis on the biological significance of various types of radiation damage and the modification of the damage by cellular repair systems.